1. Field of Invention
The present invention relates to a process for the separation-recovery of propylene and the atactic polymer from liquid propylene containing the dissolved atactic polymer (hereinafter referred to as "propylene solution" for brevity).
2. Description of the Prior Art
The so-called bulk polymerization of propylene, in which homopolymerization of propylene or copolymerization of propylene with an ethylenically unsaturated hydrocarbon monomer is carried out in suspension in liquid propylene, is well known. In this polymerization, a polymer by-product (hereinafter referred to as "atactic polymer" for brevity) is obtained in a solution of liquid propylene, after separating the isotactic polymer from the polymer slurry obtained.
Of the well-known processes for the recovery of the atactic polymer, the simplest technique is to reduce the pressure on the propylene solution to a pressure near to atmospheric pressure to thereby vaporize propylene from the propylene solution. This process enables propylene to be vaporized with only slight heating, and the atactic polymer can easily be recovered. Since, however, the process involves vaporizing a large amount of liquid propylene under reduced pressure, the resulting propylene gas must be liquefied again in an industrial-scale production, to recycle it to the polymerization step. The process, therefore, requires a compressor and becomes disadvantageous in terms of large equipment and power costs.
U.S. Pat. No. 3,644,583 discloses a process which comprises vaporizing propylene under pressure to achieve the separation-recovery of the atactic polymer, but the process has the following drawbacks: (1) It is necessary to use a fractionator equipped with many sieve or bubble type fractionation trays and fine particles of the isotactic polymer and the catalyst residue can easily adhere to the trays while supplying the propylene solution containing polymer and residue to the fractionator. In addition, the separator has a complicated structure. (2) The heat transfer coefficient of the reboiler which is used for vaporization of the propylene easily drops.
Further, Japanese Patent Publication (OPI) No. 5881/1977, filed in the names of the present inventors, discloses a process which comprises heating a solution of the atactic polymer in liquid propylene under a gauge pressure of 13 to 40 kg/cm.sup.2 thereby vaporizing propylene to recover the atactic polymer in a molten state. This process has the following advantages:
(1) By vaporizing the propylene under a pressure of 13 to 40 kg/cm.sup.2, the separated propylene gas can be easily liquefied by merely cooling it with industrial water or air; thus a compressor for the recycle of propylene becomes unnecessary.
(2) Under high-pressure conditions, in general, the vaporization of propylene is difficult because the flash effect of propylene owing to the pressure drop cannot be obtained, and besides the heat transfer coefficient of the wall surface of a vaporizer falls due to contamination of the wall surface by the atactic polymer, and the isotactic polymer and catalyst residue of fine powder, or the vaporizer is easily blocked. In the process, however, such contamination of the vaporizer walls can be decreased by first vaporizing the propylene in a vaporizer followed by separation of the atactic polymer from propylene. In this case the inside of the vaporizer is washed with the high-speed flow of liquid propylene or propylene gas. However, a reduction in heat transfer coefficient is very large in the course of a long run.